Combustion in an internal combustion engine may be initiated with an ignition spark generated from a spark plug. The ignition spark may be initiated by charging an ignition coil with a low voltage battery. The duration of the charging, or the dwell time, can determine the amplitude of the ignition coil current, and consequently the energy of the ignition spark. The energy of the ignition spark directly affects engine performance. For example, an ignition spark with lower than desired level of energy may cause unreliable combustion or misfire. On the other hand, an ignition spark with higher than the desired level of energy may increase wear of the ignition system.
Other attempts to address the issue of ignition coil control include control of the ignition dwell time based on engine operating parameters. One example approach is shown by Ruman et al. in U.S. Pat. No. 5,913,302A. Therein, ignition dwell time is determined based on engine speed and engine load.
However, the inventors herein have recognized potential issues with such systems. As one example, ignition coil temperature may affect the ignition spark energy. Variation in the ignition coil temperature may cause fluctuation in the electrical circuit resistance, which in turn may affect the ignition coil current. Therefore, in order to accurately control the ignition coil current, the dwell time may be determined based on the ignition coil temperature.
In one example, the issues described above may be addressed by a method of charging an ignition coil for a dwell time determined based on each and every of an engine temperature, an ambient temperature, and a dwell time of the most recent spark ignition. In this way, the ignition coil current may be accurately controlled by taking account of the variation in ignition coil temperature.
As one example, an ignition coil is charged with a dwell time determined based on the ignition coil temperature, wherein the ignition coil temperature may be iteratively updated with an estimated change rate of the coil temperature (e.g., coil temperature change over time, with a unit such as degrees per second). Since the ignition coil is mechanically coupled to the cylinder head, and is exposed to ambient air, the change rate of the coil temperature depends on heat transfer from the engine and the ambient air. Further, current flow within the ignition coil may heat the ignition coil internally. Thus, the change rate of the coil temperature may be calculated in real time by a controller based on each and every of an estimated heat transfer from the engine, internal resistive heating, and heat transfer from ambient air. The internal resistive heating of the ignition coil may be calculated based on the ignition coil temperature from the most recent spark ignition. The ignition coil temperature may be updated with a period shorter than the thermal time constant of the ignition coil, so that the estimated ignition coil temperature may closely track the actual coil temperature. By taking account of the heat transfer to and from the ignition coil, variation in the ignition coil temperature may be accurately tracked at any time point during engine operation without extra equipment installation. As such, the dwell time may be determined before each engine firing event based on the ignition coil temperature and an available battery voltage. In this way, the charge current in the ignition coil may be accurately controlled.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.